rcu: Size rcu_node tree from nr_cpu_ids rather than NR_CPUS

The rcu_node tree array is sized based on compile-time constants,
including NR_CPUS.  Although this approach has worked well in the past,
the recent trend by many distros to define NR_CPUS=4096 results in
excessive grace-period-initialization latencies.

This commit therefore substitutes the run-time computed nr_cpu_ids for
the compile-time NR_CPUS when building the tree.  This can result in
much of the compile-time-allocated rcu_node array being unused.  If
this is a major problem, you are in a specialized situation anyway,
so you can manually adjust the NR_CPUS, RCU_FANOUT, and RCU_FANOUT_LEAF
kernel config parameters.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

1 files changed, 1 insertions, 1 deletions

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index a4c592b66e10..0fdbc5e07302 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -2675,7 +2675,7 @@ static void __init rcu_init_geometry(void)
 {
         int i;
         int j;
-        int n = NR_CPUS;
+        int n = nr_cpu_ids;
         int rcu_capacity[MAX_RCU_LVLS + 1];
 
         /* If the compile-time values are accurate, just leave. */